1. Field of the Invention
The present invention relates to color control of printing presses and, in particular, to utilization of a linear equation to relate colorimetric density difference to solid ink density differences for ink regulation utilizing an empirically established correction matrix such that color-difference is minimized.
2. Discussion of the Related Art
Instrumental color control applied to the control of a printing press has as one of its main purposes the task of maintaining a visual match between an established color target location, which can be in the form of a customer supplied proof, or printed xe2x80x9cColor OKxe2x80x9d sheet, etc., and that same location in subsequently printed sheets such that no visually disturbing color-differences appear throughout the duration of the press run. If a color-difference exceeds the established color tolerances, then the appropriate inking adjustments are determined such that the color-difference is minimized. To determine a visual color match or color-difference instrumentally, it is a requirement that the instrument provide measurement data either directly or indirectly that are related to the spectral responses of the human eye. Such instruments include spectrophotometers and colorimeters.
In order to minimize the measured color-difference obtained from measurements using the aforementioned color measuring instrument, an equation describing the relationship between a color-difference and the appropriate control values is required. In printing, the main press control influencing the visual impression of the printed image is the inking system, which modulates the flow of ink into the press. An adjustment to the inking system in order to minimize a color difference is usually described as a density difference or an ink layer thickness difference.
For control purposes, it is highly desirable to choose a response variable that is linearly related to changes in the control variable over as large a region as possible. Although, multi-color halftone image reproduction is in general a non-linear process, under certain conditions it is possible to use linear equations to model the process by restricting the range of the transformation to a sub-region of the color gamut. Within each sub-region, which has the target color value as its origin, a set of xe2x80x9clocalizedxe2x80x9d equations can be used. The region over which the localized transformations will be linear is dependent upon the target color location and the color space used to represent the color differences used in the transformation.
Methods for performing such a procedure utilizing CIE recommended uniform color-difference parameters as the response variables are described in U.S. Pat. Nos. 4,975,862, 5,182,721 and 6,041,708.
Several aspects of the aforementioned U.S. patents can be improved upon for use in a color control system. While differences between colors described in CIE recommended uniform color space parameters, and their associated total color-difference equations, are very useful for describing color-differences, they are not the ideal color-difference descriptors from which to calculate inking control values based upon a linear model. Additionally, off-line methods of calculating the parameters of the matrix relating solid ink density differences to color-differences are not accurate enough for use in a commercial color control system. Such methods only represent the state of the system at one point in time. Dynamic methods of calculating the matrix on-line in real-time during the press run will greatly improve its effectiveness and accuracy.
The present invention provides an improved method of color control for printing presses by converting color-differences between a target area and a test area into solid ink density corrections through the use of a color space which is both colorimetric and more nearly related to solid ink density differences than the CIE recommended uniform color spaces previously used. The method maintains the use of CIE recommended uniform color parameters for determining whether or not a color correction is necessary. If a color correction is necessary, then an alternate color representation, colorimetric density, is used to determine the appropriate inking adjustments. The method establishes an increased region around a target color over which an assumption of linearity is valid. This increased linearity provides for a more accurate and robust color control method. In addition, the invention presents a robust method for calculating parameters of the linear model relating colorimetric density differences and control values on-line in real time during the press run.
The features and advantages of the present invention will be more fully understood and appreciated by reference to the following detailed description of the invention which should be considered in conjunction with the drawings.